Field of the Invention
The present disclosure relates to an image forming apparatus which forms an image on a sheet.
Description of the Related Art
Conventionally, some image forming apparatuses which form an image on a sheet include a mechanism for aligning a position of a sheet with an image to be written on the sheet. In this mechanism, for example, the sheet is shifted in a direction (a width direction of the sheet) which is perpendicular to a sheet conveying direction, thereby correcting the sheet position. Thus, there are image forming apparatuses which include a sheet conveyance device having a mechanism for correcting the posture (skew) and the position of the sheet to be conveyed to an image forming unit. Further, with the tendency to improve productivity of an image forming apparatus in recent years, a period allowed for a shift action by the registration roller for a sheet position correction and that allowed for the process of a return operation after a shift etc., to spend becomes short. Therefore, when a position deviation of the sheet conveyed toward the image forming unit is large, the amount the shift action for correcting the same also becomes large. Therefore, it may not possible to complete the return operation within a predetermined period in some cases.
Further, it is desirable to minimize the shift amount at the time of correcting the position deviation. For example, as the shift amount of the registration roller becomes large, “twist” may arise in the sheet. As a result, the conveying direction of the sheet may be inclined, or shift accuracy may be deteriorated. It is desirable to minimize the shift amount of the sheet, also from this point of a view. In addition, as the miniaturization of an apparatus, there is a tendency to decrease the distance between the registration roller which has a shift mechanism and the conveyance roller positioned at the upstream thereof (i.e., the upstream in the conveying direction). And, upon shift action by the registration roller, for all the conveyance rollers except for the registration roller, it is necessary to cancel nipping of the sheet by the conveyance rollers. Therefore, it may disturb the minimization of an apparatus, and may increase the cost due to a complicated configuration of the apparatus.
Under these circumstances, US2009/0154975 (A1) describes an image forming apparatus which employs a control method in which, based on the detection result of a certain sheet in a width direction, an image forming position of a sheet, which is conveyed after a predetermined pages, is determined for decreasing the shift amount. Specifically, the mechanism provided in the apparatus corrects the inclined tip portion of the sheet by moving the tip of the sheet conveyed by the pre-registration roller to contact with the nip portion of the registration controller, which is provided upstream of the registration roller, to curl the sheet. Further, the apparatus includes a width direction correcting mechanism which corrects the position in a direction which is perpendicular to the conveying direction of the sheet (i.e., width direction of the sheet). This width direction correcting mechanism includes a width direction detection unit configured to detect a position in a sheet width direction provided downstream of the registration roller, and shift unit configured to shift the registration roller in the sheet width direction with nipping the sheet.
In recent years, an intermediate transfer tandem type in which image forming units each corresponding to one of four colors are arranged on an intermediate transfer belt is mainly used, due to its advantageous adaptability for a variety of sheets and advantageous print productivity. However, especially in the color image formation, a peripheral length of an intermediate transfer belt is relatively long, therefore, it takes relatively long time from a completion of forming an image of first color to an arrival of the image to a secondary transfer section.
On the other hand, US2009/0154975 (A1) describes an image forming apparatus in which a sheet width direction detection unit is arranged downstream of a registration roller. Therefore, the time from detecting a sheet end to a conveyance of the sheet to a secondary transfer section is less than the time for the arrival of the image to a secondary transfer section describe above.
Thus, in this image forming apparatus, as to each of the first sheet and the second sheet, the correction amount of the image formation position is decided to be zero, while the deviation amount of the position of the sheet end itself is used as the amount to be shifted by the registration roller. As above, there remains a problem that the shift amount of the registration roller becomes large in the first sheet, for example.
The present invention is directed to solve these problems in the prior art, and is mainly directed to an image forming apparatus which can minimize, from the first sheet as well as following sheets, the shift amount by the registration roller. Particularly, the present invention provides an image forming apparatus for minimizing the shift amount even in a configuration in which the conveyance distance of the sheet in an image process becomes relatively long, for example, in an intermediate transfer tandem type.